Insights into the origin of the tendency of the PI3-SH3 domain to form amyloid fibrils.

The SH3 domain of the p85alpha subunit of phosphatidylinositol 3 kinase has been found to form amyloid fibrils in vitro under acidic conditions. PI3-SH3 is peculiar due to a large insertion of 15 amino acid residues in the n-Src loop when compared with more canonical members of the family. Spectrin-SH3 (SPC-SH3) with a shorter loop does not form fibrils under any of our conditions tested. Thus, it could be that the longer loop could play a role in amyloid formation. To investigate this we have engineered two chimeras containing the common core of the PI3-SH3 and SPC-SH3 with an exchanged n-Src loop. Thermodynamic and kinetic analyses show that the two chimeras are less stable than the parent proteins, but useful for our comparative purposes they have similar stability. Neither stability, nor folding rates, or pH transition can be invoked as being responsible for the amyloid formation in the PI3-SH3 domain. Substitution of the long n-Src loop in PI3-SH3 by that of SPC-SH3 does not prevent fibril formation. The SPC-SH3 with the PI3-SH3 n-Src loop is in an A-state at low pH and forms beta-sheet amorphous aggregates, but not amyloid fibrils. Thus, we conclude that, for a protein to form ordered fibrils, a delicate balance between solubility of non-native states to allow efficient nucleation and the formation of amorphous aggregates, must be achieved. It is the amino acid residue sequence of the protein and probably its parts that play a determinant role in shifting this balance in one direction or the other.